Conventional resonator fiber optic gyroscopes (RFOG) require at least two light beams, one transmitted in the clockwise (CW) direction of the resonator, and the other launched in the counterclockwise (CCW) direction. Their frequencies are locked to the CW and CCW resonant frequencies, and are used to measure rotation rates. The relative (differential) frequency noise between the CW and CCW light will contribute to the overall noise in the rotation rates. Differential frequency noise can be reduced by phase locking the CW and CCW lasers using a phase-locked loop (PLL).
Although phase-locked lasers exhibit very low differential frequency noise immediately at the laser output, differences in thermal and acoustic noises experienced by the lasers in the CW and CCW optical paths to the resonator de-correlates the two beams at the input to the resonator, creating differential phase noise and causing degraded RFOG performance.
Since the noise experienced in the CW and CCW optical paths are different, common mode subtraction has a limited effect toward removal of this noise.